1. Field of the Invention
This invention relates to radio navigation beacons, particularly those employed in the so-called Doppler radio navigation systems.
2. Description of the Prior Art
In British patent specification No. 1,234,541 (to mention one typical prior art description of a beacon for Doppler navigation) there is described a type of linear array beacon in which a source of radio frequency is commutated to separate radiator elements in order to simulate bidirectional constant velocity motion of the source. Navigational information is derived from these beacons, e.g., elevation angle from a vertical array and azimuth angle from a horizontal array, as the Doppler shift of frequency radiated from the array (as observed at a remote receiving station, i.e., an approaching aircraft) this receiver station subtends to the line of movement of the source.
The radio frequency of the transmitter is typically at least 1 GHz, and since the maximum Doppler frequency shift is of the order of KHz, it is necessary, in practice, to use a reference antenna, at the beacon, which radiates a second radio frequency slightly offset from the commutated frequency, e.g., by 20 KHz and constituted by an upper or lower sideband of the commutated frequency. As described in the above-referenced British patent specification No. 1,234,541, the offset frequency alternates between upper and lower sideband according to direction of scan in the "to-and-fro" scanning which makes up the bidirectional movement (commutation direction) of the source.
The Doppler shift to the frequency of the moving component is then detected as a charge on the beat frequency between the moving component (or main bearing signal) and the reference signal. Thus, the indicated change of beat frequency, which bears the navigational information, is determined by the difference between the two paths.
With the fixed reference antenna of the beacon described in British patent specification No. 1,234,541, this change of path difference arises solely from the movement simulated by the commutated array.
The desired signal components are those transmitted by direct propagation from beacon to receiver, but there are inevitable also generated unwanted, so-called multipath components, by reflection in any practical situation.
Particularly with a horizontal, azimuth guidance array and at very low altitudes, a multipath situation may occur, by signal reflection from a buidling such as a hangar, which affects the integrity of the reference component. If a multipath signal occurs at substantially the same strength and at a 180.degree. phase with respect to the reference component from a single reference antenna, the reference component will, at least temporarily, disappear. This would result in loss of azimuth guidance at a crucial point in the landing approach of an aircraft relying on the ground beacon transmissions.